Conventionally, hydrogen may be stored in the form of metal hydride powders. During the hydriding/dehydriding cycle, strain behavior causes the particle bed to become unstable, resulting in settling and compacting of the particle bed. Through repeated cycling, the three-dimensional relationship of the powder particles continues to change, causing strain to continue to increase. When using metal hydride powders, inefficient heat transfer can hamper the rate and effectiveness of the hydriding/dehydriding cycle.
When using traditional metal hydride powders, safety and handling issues arise as many materials are pyrophoric, or become pyrophoric once contacted with hydrogen. In addition, powders may be blown into the hydrogen stream, which requires complicating filtering and also introduces a pressure drop into the fuel system.
The hydriding/dehydriding process imparts strain on the storage medium causing it to expand during charging and contract during discharge. This strain, which can be quite significant, is conventionally dealt with by designing a hydride storage vessel with expansion room to accommodate the strain. However, the unstable nature of the particle bed causes the packing of hydride material, effectively filling up the expansion room and causing significant strain to be exerted on the walls of the storage vessel. Therefore, the storage vessel must be designed to deal with this internally induced mechanical strain, either by increasing wall thickness or developing a system of internal structures to cause the bed to ‘unpack’ itself when straining occurs. The need for these complicated designs of a storage vessel effectively reduces the volumetric hydrogen storage density of the metal hydride powders.
The hydriding/dehydriding process causes the powder particles to pack more tightly, thus increasing the compaction of the system. The three-dimensional relationship of the particles changes throughout the hydriding/dehydriding cycle, negatively affecting the hydrogen storage ability of the powder.